A package of the type with which the present invention is concerned must meet stringent requirements. It must be capable of supporting an optical fibre (that contains the Bragg grating) whilst the optical fibre is strained to a level approaching 100 grams. The strain is imposed for the purpose of tuning the grating to a required centre wavelength, and the strain must be maintained at a substantially constant level for the service life of the grating. This requires that the optical fibre be anchored securely within the carrier, that the anchoring exhibit stability under varying temperature and humidity levels, and that the package exhibit dimensional stability with changes in ambient temperature.
One package that has been developed in an attempt to meet these requirements is disclosed in International patent application number PCT/AU98/00473, dated Jun. 18, 1998. The package comprises a temperature compensating carrier in which an in-fibre grating is anchored by deposits of epoxy resin.
This package has been found to provide long term stability under optimum working conditions, but it does exhibit slight sensitivity to changes in humidity levels, this flowing from inherent characteristics of the epoxy resin. An increasing humidity level causes a downward shift in the centre wavelength of the grating, this making the package unsuitable for use, for example, in high channel count DWDM systems.
Various epoxy compounds have been evaluated in an attempt to resolve the humidity-induced problem, but none has shown any significant benefit over another.
Alternative approaches have been taken, using metal solder to anchor metal-clad optical fibre to supports. However, these approaches have resulted in packages that exhibit significant levels of creep over time.
The present invention has been developed in an attempt to avoid the above described problems and is based on the use of glass solder to provide for stable, humidity insensitive anchoring of optical fibre to supports for the optical fibre.
The selection of glass solder as the anchoring medium has led to the recognition that the supports should have a coefficient of thermal expansion (“CTE”) that is higher than that of the glass solder, in order that compression might be induced in the glass solder during post-fusion solidification and simultaneous cooling of both the supports and the glass solder. Furthermore, it has been recognised that portions of the optical fibre that are bonded to the supports by the glass solder should be wholly embedded within the glass solder, and that the glass solder itself should be wholly contained (and hence restrained) within channels in the supports.
Various prior art references disclose the use of glass solder for hermetically sealing, bushing and terminating optical fibres and other photonic devices. In these contexts reference is made to U.S. Pat. Nos. 4,904,046, 5,143,531, 5,177,806, 5,337,387, 5,509,952 and 5,664,040. Also, U.S. Pat. No. 5,682,453 discloses the use of glass solder for bonding together first and second glass-based optical elements.
Perhaps more significantly, U.S. Pat. No. 5,694,503 discloses an in-fibre Bragg grating that is packaged to provide for reflectance that is substantially temperature independent. This package incorporates an Invar support member to which a length of optical fibre is secured by glass solder and epoxy bonding material. However, the support member and associated bonding platforms are formed from a material having a CTE that is equal to or less than that of the glass solder.